Wireless communication systems use antennas to communicate signals. A wireless local area network (WLAN) is a type of wireless communication system that communicates information between nodes in a given area.
Types of Signals
Narrowband and Wideband Signals
Most current wireless communications systems are narrowband signal systems. Narrowband signals have signal bandwidths typically ranging from tens of kilohertz (kHz) (e.g. 50 kHz) to hundreds of kilohertz (500 KHz). In contrast, wideband, or broadband, signals have signal bandwidths greater than 1 MHz.
802.11 and 802.11a
One type of wideband signal is the signal used in WLANs using the Institute of Electrical and Electronic Engineers (IEEE) 802.11 standard. The IEEE 802.11 standard (802.11) outlines Media Access Control (MAC) and Physical Layer (PHY) specifications for WLANs. The IEEE 802.11a standard (802.11a) is a part of 802.11 and addresses communications in high data rate wideband packetized wireless communication systems, covering frequencies of operation between 5 GHz and 6 GHz. 802.11a uses orthogonal frequency-division multiplexing (OFDM) modulation, which allows communication to occur at very high data rates by transmitting data over multiple frequency bins over a wide frequency range. Discussions herein applicable to 802.11a are also applicable to IEEE 802.11g. The IEEE 802.11g OFDM standard is the same as 802.11a, with the exception of operating in the 2.4 GHz band. 802.11 takes into account the successful and unsuccessful transmission of packets and includes mechanisms designed for dealing with packet transmission problems. 802.11a wireless communications systems and other wireless communication systems can experience numerous problems during the transmission and reception of signals.
Circuit Impairments
For example, wireless communication systems can encounter problems with circuit impairments in their receiver circuits. In particular, receiver circuits can experience the following circuit impairments: (1) frequency offset; (2) direct current (DC) offset; (3) carrier phase offset, and (4) timing offset.
A typical prior art receiver circuit 100 is depicted in FIG. 1A. Receiver circuit 100 includes an antenna 110, an analog front end 120, and a baseband system 130, logically interconnected as shown in FIG. 1A. Analog front end 120 includes a local oscillator 122, a low noise amplifier (LNA) 123, a mixer 124, analog amplifier 125, and analog filters 126. Baseband system 130 includes an analog-to-digital converter (A/D) 132 and a digital signal processor (DSP) 134. The non-idealities in the components of analog front ends and baseband systems, such as the non-idealities in local oscillator 122, mixer 124, filters 126, A/D 132, and DSP 134, provide the circuit impairments that would be encountered by receiver circuits, such as receiver circuit 100.
Prior art receiver circuits attempt to correct for circuit impairments with circuit impairment cancellation circuitry. For example, in FIG. 1B, prior art receiver circuit 140 includes a modified baseband system 150 logically coupled to analog front end 120. Modified baseband system 150 includes a circuit impairment cancellation unit 152 logically interconnected between A/D 132 and DSP 134. Circuit impairment cancellation unit 152 estimates the circuit impairments from the digital output of A/D 132. Then, circuit impairment cancellation unit 152 cancels the circuit impairments in the signals from A/D 132.
Channel Effects—Fading and Multipath Communication Channels
For example, a wireless communication system could encounter channel effects, such as transmitting signals across a fading communication channel. The fading in the communication channel may be caused by mutipath and propagation loss.
In the case of multipath channel, the RF energy that is transmitted between transmit and receive antennas experiences destructive and constructive interference due to multiple paths taken by the RF energy with multiple delays on the way to a receive antenna. Such multipath interference modulates the phase and attenuates the amplitude of signals across all frequencies and carriers used by a wireless communication system. In a WLAN, such multipath interference could cause a receiver to receive a packet in error or to miss a packet entirely.
Prior art receiver circuits attempt to correct for channel effects, such as fading channels and multipath interference, with channel correction circuitry. For example, in FIG. 1C, prior art receiver circuit 160 includes a modified baseband system 170 logically interconnected with analog front end 120 and a decoder 176. Modified baseband system 170 includes a channel correction unit 172 logically interconnected between A/D 132 and decoder 176. Channel correction unit 172 performs channel equalization on the output of A/D 132 for narrowband signals.
Antenna Diversity
Prior art receiver circuits attempt to correct for channel effects, such as fading channels and multipath interference, with antenna diversity. In a wireless communication system with antenna diversity there is a multiple antenna receiver A receiver with multiple antennas is used so that in the event of poor signal reception due to a fading channel on one antenna, a good channel with no fading will likely exist on another antenna. For example, in FIG. 1D, prior art multiple antenna receiver 180 includes multiple antennas 181, 182, the receive chain 183, and a diversity switch 189, logically interconnected as shown. Receive chain 183 includes an analog front end 185 and a baseband system 187. Analog front end 185 could be like analog front end 120, and baseband system 187 could be like baseband system 130. When a particular communication channel is fading, diversity switch 189 switches from one antenna to another antenna in order to obtain a communication channel that is not fading. Unfortunately, diversity switch 189 causes switch loss in received signals. Moreover, switching diversity provides limited diversity gain, since only the signal of the selected antenna is used at receiver. Whereas, optimal combining of the signals from the antennas would result in greater diversity gain.
Fast Antenna Switched Diversity
Prior art fast antenna diversity techniques have been used to manage multiple antennas. For example, in a fast antenna diversity communication system with two antennas, when a packet arrives, a first antenna is used to receive the signal. After receiving the signal for a sufficient period of time to judge reception quality, the communication system switches to a second antenna. The second antenna is then used to receive the signal until the quality of reception can be judged. Finally, the system switches to the antenna with the best reception. In some cases, more than two antennas are used in a fast antenna diversity communication system.
Trying and testing multiple antennas using fast antenna diversity typically takes place during a preamble, header, or training portion of the packet being received. The preamble is examined rather than the data so that no data is lost while the different antennas are being tested.
Problems with Fast Antenna Diversity and 802.11a
Fast antenna diversity is undesirable for 802.11a signals and for other high data rate wireless communication signals for several reasons.
Poor Estimation of Channel Quality
First, the packet length of 802.11a signals and other high data rate wireless communication signals leads to a poor estimation of channel quality with fast antenna diversity techniques. For example, the packet preamble in a 802.11a signal is quite short at eight microseconds total duration. A Short preamble is desirable in any high data rate communication system in order to keep the efficiency of the communication system high. As data rates increase, the duration of packets tend to decrease.
Degradation of Communication Performance
In addition, fast antenna diversity degrades the performance of 802.11a and other high data rate wireless communication systems. Time that is consumed in switching and measuring the signals from different antennas reduces the amount of time available to perform other functions that commonly need to be performed during the packet preamble in 802.11a signals and other high data rate wireless communication signals. These functions may include (1) correctly setting the gains of amplifiers in a receive chain, (2) extracting the frequency offset of a received signal, and (3) finding proper symbol boundaries for determining symbol timing. When the preamble is short, the quality of the frequency offset, gain setting, or symbol timing could be compromised if time is spent selecting the best antenna. Therefore, forcing antenna selection into the time of the preamble would degrade the overall performance of high data rate wireless communications systems, such as 802.11a systems.
Difficulty in Detecting Differences Among Antennas
Also, fast antenna diversity switching during the packet preamble creates an additional challenge for wideband signals such as 802.11a OFDM signals. The preamble does not have the frequency resolution to identify narrowband notches in the received signals. Therefore, the preamble can not be used to sense many of the narrow notches within the narrow frequency bands that could occur as a result of multipath interference with wideband signals. A switching decision only based on the preamble power, could cause switching to an antenna with a frequency domain notch, and hence loss of the packet.
An additional challenge for detecting differences among the channels during the packet preamble for certain wideband and signals, 802.11a OFDM signals in particular, is that the very small duration of the combined Short and Long training symbol sequences, and in particular the very limited duration of the Short training symbol sequence. Due to this short duration, which provides the desired period of time during when a decision on which one of many different antennas is best to use must be made, conventional techniques that require longer period of time to make such decisions cannot be used.
Combining Signals
Combining the antenna signals is another diversity method. The antenna signals have to be co-phased first and then combined, in order to achieve the coherent combining gain. This task is easier when signals are narrowband and more challenging for wideband signals.
Combining Narrowband Signals
In a narrowband signal wireless communication system, two or more receive signals from two or more antennas generally do not show significant variations across the frequency band (i.e., the signals have a relatively flat response). Thus, the two or more narrowband signals can be coherently combined rather easily using an antenna diversity combining technique with little risk of either (1) losing information by deviating from the true signal or (2) the received signals canceling each other out. Generally, the amplitude and phase responses of narrowband signals do not vary as significantly across the frequency band as the amplitude and phase responses of wideband signals, such as 802.11a signals. Hence, the combining weights for narrowband signals are not frequency dependent and narrowband signals from different antennas can be easily phase corrected and combined.
Problems with Combining Wideband Signals
In contrast to narrowband signals, combining wideband wireless signals is much more complicated via traditional combination methods or conventional narrowband diversity techniques if they are to overcome frequency selectively because of the wide variations in the phase and amplitudes of the signals across the wide frequency bandwidth.
Antenna Diversity Combining
Several conventional antenna diversity combining techniques exist. Many of these techniques are based on examining a combination of signals from two or more antennas. One combining method is maximal ratio combining (MRC) where signals coming from two or more antennas are cophased and weighted proportionally to their signal-to-noise ratios (SNRs) and are added together to form a weighted combination signal. MRC results in optimal SNR improvement, where the combined signal SNR is equal to the sum of SNRs for each antenna signal.
Another combining method is Equal Gain Combining (EGC). In equal gain combining, weights with same magnitudes and different phases are used for all signals. Referring to FIG. 1E, prior art equal gain combiner 190 includes analog front ends 191, 192, baseband units 194, 195, phase correction units 196, 197, and the summer 198, logically interconnected as shown. However, EGC's performance is suboptimal, where the combined SNR is typically higher than each antenna SNR, but smaller than the sum of SNRs.
Such prior art antenna diversity combining techniques may work well for narrowband signals, where the phase and weights are not frequency dependent. However, the conventional techniques do not work well for wideband signals that have received phase and power that are not constant over the received signal bandwidth and that are frequency dependent, such as 802.11a signals. Therefore, conventional antenna diversity combining techniques are not applicable to wideband signal wireless communication signals, such as 802.11a signals.
Therefore, a cost effective and efficient multiple antenna receiver antenna combining technique that is suited to confront the challenges posed by high data rate wideband packetized wireless communication signals, such as 802.11a signals, and that implements frequency dependent weighting in combining such signals is needed. Thus, the present invention provides an efficient and low cost system and method of multiple antenna receiver combining of high data rate wideband packetized wireless communication signals.